1. Technical Field
The present invention relates generally to an impedance control circuit of a semiconductor device for detecting a characteristic-impedance of a transmission line and matching the impedance to the impedance of an output driver and an on chip terminator. In particular, the present invention is directed to a programmable impedance control circuit and method for controlling the impedance in a controlled, programmable manner, irrespective of a variation of an external impedance subsequent to a locking operation in which an internal impedance of an integrated circuit (IC) chip is set to the external impedance, thereby maintaining a reliable and stable system.
2. Description of the Related Art
Recently, the use of various xe2x80x9con-chipxe2x80x9d termination techniques have been employed for use in interface circuitry for high-speed data transmission in digital circuit designs. Such termination techniques include, for example, series termination, source termination, and parallel termination, and each of these techniques has advantages and disadvantages associated therewith. For instance, one advantage of series termination (which comprises a resistor between the output of a driver and the interface line) is that its termination resistor consumes less power than all other resistive termination techniques. With parallel termination (which comprises a resistor connected between one end of a transmission line and ground (or voltage source VCC)), one advantage is that good signal-integrity is maintained, although the swing level of a signal may be lowered (i.e., the high-output level of the signal may be degraded) due to minor dc power dissipation in the termination resistor. With a source termination approach, the impedance of the transmission line is matched by the impedance of a source driver.
One method for employing on-chip termination comprises transmitting (in full swing) data through a transmission line wherein an output driver performs source termination and a receiver performs parallel termination. To implement this method, it is desirable that the output driver and the on-chip termination be implemented with a resistor. With this approach, however, because the output driver and the on-chip termination are preferably disposed within a chip, the desired termination cannot be obtained when the characteristic impedance of the transmission line varies. Thus, the circuit should be constructed such that a desired value for the characteristic-impedance could be set in a program. For this purpose, an impedance control circuit is utilized to detect a characteristic impedance of a transmission line and transmits the result to the output driver and the on-chip termination.
A programmable impedance control circuit serves to match an internal impedance of a chip (e.g., impedance of an output driver) according to a resistance value of an externally connected resistor, for example. In addition, the circuit provides a mechanism for matching an internal impedance to an external impedance by actively updating a digital code during a predetermined period in response to variances in voltage and temperature.
FIGS. 1a and 1a are diagrams that illustrate a conventional impedance control circuit and its operation characteristics, respectively. The conventional impedance control circuit of FIG. 1a employs a scheme (as illustrated in FIG. 1b) in which a locking operation is implemented when an external resistance value is uniformly maintained (a first steady state occurs) and a tracking operation is implemented to actively follow variations in the external resistance value after the locking operation to adjust the internal impedance. One problem associated with the conventional scheme is that the impedance may be abruptly changed even at the constant external resistance value when the tracking operation is actively performed in response to any changes in the external conditions (voltage, or temperature) after a locking operation. Such an abrupt change in impedance causes a failure in its entire system operation.
During normal operation of the input termination and output driver, however, the voltage and temperature typically do not abruptly vary from system condition under which the input termination and output driver were set (i.e., locked). Therefore, the possibility that the programmable impedance control circuit instantaneously may fail in its operation in response to an external noise is not significant. Accordingly, a system and method for controlling the impedance in a programmable manner, irrespective of variations of the impedance of an external coupling (e.g., transmission line) is highly desirable. Indeed, it would be considered a safe transmission method in the entire system either to have the impedance code xe2x80x9clockedxe2x80x9d in the original operation and continuously maintained without a tracking operation during operation of the chip, or to track a regular code, by one or two codes out of the initial locked codes, during a code updating after setting of codes, even if the tracked codes change significantly.
Accordingly, in order to solve the aforementioned problems, an object of the present invention is to provide a programmable impedance control circuit by which a locking operation is first implemented to set an internal impedance (e.g., impedance of an output driver) to match an external impedance (e.g., characteristic impedance of a transmission line) during a first mode of operation in the system environment and wherein a programmable tracking operation is performed to adjust the internal impedance to the external impedance according to a preprogrammed protocol.
Another object of the present invention is to provide a programmable impedance control circuit by which a preprogrammed protocol for tracking comprises continuously maintaining a locked code value after the locking operation irrespective of detected changes in the external impedance, or by which tracking is performed subsequent to the locking operation by adjusting the internal impedance by no more than a predetermined number of code values from the initial locked code value in response to changes in the external impedance during operation of the chip.
Still another object of the present invention is to provide a programmable impedance control circuit in which the impedance of transmission line is controlled in a programmable way by separating a locking operation and a tracking operation using digital code or time.
In one aspect of the present invention, a method for providing impedance control to match an external impedance to an internal impedance of an integrated circuit chip comprises the steps of:
detecting an impedance control signal indicative of an external impedance value;
performing a locking operation to detect a locked state and output a corresponding locked code value to set an initial internal impedance, upon detecting a first occurrence of a steady code sequence in the impedance control signal for a first predetermined cycle; and
performing a preprogrammed tracking operation in response to a variation of an external impedance subsequent to the locking operation.
In another aspect, a preprogrammed tracking operation comprises maintaining the locked code value irrespective of variations of the external impedance subsequent to the locking operation.
In yet another aspect, the preprogrammed tracking operation comprises adjusting the value of the initially set internal impedance by no more than a predefined number of code values from the locked code value.
In another aspect, the preprogrammed tracking operation comprises performing the tracking operation at a clock frequency that is lower than a clock frequency at which the locking operation is performed.
In yet another aspect, the preprogrammed tracking operation comprises maintaining the locked code value and then changing the locked code value to an updated code value after detecting an occurrence of a steady code sequence in the impedance control signal for a second predetermined cycle subsequent to the locking operation.
In another aspect, the impedance control method is preferably implemented for matching the characteristic impedance of a transmission line to the impedance of an output driver.
In still another aspect of the present invention, a programmable impedance control circuit comprises:
a first circuit for processing a plurality of reference voltages to generate an impedance control signal and for performing a locking operation to detect a locked state and output a corresponding locked code value upon detecting a first occurrence of a steady code sequence in the impedance control signal for a first predetermined cycle; and
a second circuit for storing the locked code value output from the first circuit and for performing a preprogrammed tracking operation in response to a variation in an external impedance subsequent to the locking operation.
These and other objects, features and advantages of the invention will become more apparent from the following detailed description of the preferred embodiment made with reference to the accompanying drawings: